The present invention relates to the field of optical scanners. More specifically, in one embodiment the invention provides a method and apparatus for improving the quality of images resulting from the optical scan of a document, and, more particularly, a method and apparatus for increasing the contrast of a document to be scanned either for optical image reproduction or for optical character recognition, using an optical filter screen placed between the scanning mechanism and the document to be scanned.
An optical scanner is a well-known device that uses a light source and a system of reflective mirrors and light sensors to read an image contained on an original document, then convert that image to electrical signals that a computer can interpret. Optical scanners have two primary uses: for reproducing text and for reproducing graphics.
Scanning graphics and text dramatically reduces printing costs and reproduction time. On an average, by-hand paste-ups may cost up to 50 times as much as computer graphics. However, certain drawbacks exist when using a scanner. Image quality is not always sharp, especially if the document or artwork has very fine lines or type. When scanned, these lines often appear to have an uneven consistency or "choppy" effect. Further, the recognition of alpha-numeric characters is limited by the ability of an optical scanner to distinguish between the character forms on the document and the blank background of the document. This becomes a problem where the characters are printed lightly on the page, or where the edges of the characters are vague or fuzzy.
One attempted solution to these problems has been the use of high-resolution scanners. High-resolution scanners are better capable than low- or moderate- resolution scanners of detecting small areas, and of resolving areas of gray into details. Further, high-resolution scanners are less likely to be "fooled" into taking image imperfections in the original document as part of the image itself, thus improving the accuracy of the scan. However, this solution is a slow or sometimes highly costly one, and is not available to the owner of a low- or moderate- resolution scanner, short of changing hardware altogether.
A second solution is the use of image-manipulation programs (software) to improve image quality. However, image-manipulation programs tend to be expensive and time-consuming. Manipulation of a scanned image using software may also result in distortion of the patterns of dots forming the image, creating a wave or circular "moire" pattern in the resulting image. Further, most scanning software manipulates or enhances an image after it has been scanned. Reading in a low quality image gives the image-manipulation software a less-than-optimum image to work with. It is much better if the scanned image is the highest quality it can be while it is being scanned.
A third attempted solution is to change the light source to reduce the amount of excess light in the scanner. The most common source of light in a scanner is a quartz halide lamp. Quartz halide provides a very intense white light, and produces truer "whites" and better contrast than other sources of light. However, quartz halide also emits a great amount of excess reflected and refracted light, causing inaccurate scans. It has been tried to substitute fluorescent light for the quartz halide source, which provides a less intense, "softer" illumination, reducing the amount of excess refracted light. However, the fluorescent light has a bluish-green cast that significantly reduces contrast between dark and light areas and brings the whites and blacks down to more moderate levels of gray tones. Thus, attempts to modify the light source to provide for high contrast in addition to an acceptably low level of excess light, have not produced satisfactory results.
Thus, while prior art systems for improving the quality of scanned images have met with some success, problems remain. Accordingly, it is seen that a method and apparatus for improving the accuracy of reproduction of a scanned image at the time of scanning by increasing contrast and decreasing the amount of excess light, and without increasing the resolution of the optical scanner, is desired.
It has been known in photography that certain enhancements in black and white images may be obtained with different-colored filters attached to a camera lens.